1. Field of the Invention
The present invention relates to a method of forming an isolation region that is used for manufacturing a semiconductor integrated circuit.
2. Description of the Related Art
In a semiconductor integrated circuit including a multiplicity of devices formed on a single piece of semiconductor material, namely, a semiconductor substrate, isolation regions are formed between device regions to electrically isolate the respective devices. There are a variety of known methods of forming the isolation regions. For instance, according to a method called a LOCOS (LOCal Oxidation of Silicon) method, the formation of the isolation region involves, at first, forming a pad oxide layer (SiO.sub.2) and a silicon nitride layer (Si.sub.3 N.sub.4) on a silicon substrate. Subsequently, an opening is formed in a portion, corresponding to the isolation region, of the silicon nitride layer by use of photo lithography and an etching technique. Thereafter, the silicon substrate is thermally treated in wet oxygen, and thick SiO.sub.2 (field oxide) is formed in an area (the isolation region), corresponding to the opening of the silicon nitride layer, of the silicon substrate. That is, according to the LOCOS method, the isolation region is formed by the selective oxidation of the silicon substrate, wherein the silicon nitride layer is used as a mask.
According to this LOCOS method, the isolation regions can be relatively easily formed. Based on the LOCOS method, however, the oxidation progresses not only in a thicknesswise direction of the silicon substrate but also in a crosswise direction, and consequently there is formed a field oxide assuming such a configuration as to intrude beneath the silicon nitride layer in the vicinity of the opening. That is, according to the LOCOS method, the isolation region is formed with a larger width corresponding to an oxide (known as a bird's beak because of its configuration) formed by the crosswise oxidation, than the width of the isolation region defined by a treatment of the silicon nitride layer.
Therefore, only an isolation region having an isolation width larger than a given value can be formed by the LOCOS method alone, and, if an isolation region having a narrow isolation width is required to be formed, the isolation region is formed by using a so-called trench method in combination therewith. In this case, to begin with, a isolation region having a large isolation width among the isolation regions that should be formed is formed on the silicon substrate by employing the LOCOS method. Subsequently, a trench is formed in a portion of the silicon substrate, where an isolation region having a narrow isolation width, of the silicon substrate. Then, silicon oxides are deposited on the silicon substrate by a CVD (Chemical Vapor Deposition) method so that the relevant trench is filled with silicon oxides (SiO.sub.2). Thereafter, the same silicon oxides are etched back, thereby forming a structure in which the silicon oxides are left within only the trench.
Thus, if the LOCOS method and the trench method are employed in combination, it is possible to form an isolation region having a narrow isolation width and an isolation region having the larger isolation width. However, the isolation regions based on the two methods must be formed sequentially, and hence a problem arises, wherein a greater amount of time than in the case of forming an isolation region by use of only one of the two methods, is needed. Note that the trench method entails a difficulty of filling the interior of the trench having a larger width with the silicon oxides. Hence, forming the isolation region having the narrow isolation width and the isolation region having the larger isolation width by only the trench method is not practiced.
Desired therefore is a method capable of forming the isolation regions having a variety of isolation widths in a short period of time (by a small number of steps). A method called a high-pressure oxidation method is proposed as one candidate for such methods.
The high-pressure oxidation method is a method classified as a LOCOS method. According to the high-pressure oxidation method, after a mask has been composed of the silicon nitride layer, the silicon substrate in oxidized in dry O.sub.2 at a pressure on the order of 25 atm. at a temperature of approximately 1000.degree. C. According to this method, the oxidation progresses in a vertical direction faster than the oxidation in the crosswise direction, and therefore field oxide with a small bird's beak can be formed. Incidentally, for details of oxidation conditions based on the high-pressure oxidation method and a corresponding relationship between a characteristic and a configuration of the field oxide to be formed, reference is made to pp.525-527, Extended Abstracts of the 1993 International Conference on Solid State Devices and Materials, Makuhari, 1993, and pp.899-901, Extended Abstracts of the 1995 International Conference on Solid State Devices and Materials, Osaka, 1995.
As stated above, when using the high-pressure oxidation method, field oxide with the small bird's beak can be formed. Hence, it follows that the isolation region having the narrow isolation width can be formed together with the isolation region having the large isolation width. However, if the isolation region is formed simply by using the high-pressure oxidation method, a much steeper stepped portion than in the case of forming the isolation region by suing the LOCOS method and the trench method in combination, is formed at a boundary between the isolation region and the device region.
That is, when forming the isolation region by employing the LOCOS method and the trench method in combination, the field oxide (the isolation region) formed based on the LOCOS method comes to have a comparatively large bird's beak, and hence there is no possibility to form a portion of the field oxide with an abrupt fluctuation in height at the boundary between the device region and the isolation region. Further, when forming the isolation region having the narrow isolation width, the etch-back process is carried out, and therefore a portion with an abrupt fluctuation in height is never formed in the periphery of this isolation region.
Contrastingly, when forming the isolation region based on the high-pressure oxidation method, the field oxide is formed without a large bird's beak, and consequently a portion with a relatively abrupt fluctuation in height is produced at the boundary between the isolation region and the device region.
Thus, in the case of forming the isolation region by the high-pressure oxidation method, there must be formed a structure including a steep stepped portion on the surface thereof. As a result, some problems (a disconnection of wire, and formation of etching residue occur when performing a variety of treatments after forming the isolation region.